This invention relates to a vehicle air conditioning system, and more particularly to a control for maintaining a predetermined level of outside air in a system capable of mixing outside air with recirculated air and having a controllable air conditioning blower motor.
A vehicle air conditioning system performs two primary functions: temperature regulation and dehumidification. After an initial cool-down period where the air inside the vehicle is relatively hot, these functions and the vehicle fuel economy can often be enhanced by drawing at least a portion of the air supplied to the cabin space from the cabin itself instead of from outside the cabin. The introduction of air drawn from the cabin generally reduces the enthalpy and moisture content of the inlet air mixture to be conditioned for redelivery to the cabin. In manually controlled systems operated in a xe2x80x9crecirculationxe2x80x9d mode, the introduction of outside air is commonly accomplished through the use of a mechanical bleed device designed to maintain a designated proportion of outside air under a specific set of static circumstances (for example, at a given vehicle speed and blower motor setting). In other manually controlled systems and some automatically controlled systems, inlet air mixing is achieved with an inlet air mixing device, such as controlled door in an inlet air duct. In such cases, the inlet air mixture comprising predominantly outside air when the system is operated in an xe2x80x9coutside airxe2x80x9d mode, and predominantly air drawn from inside the cabin when the system is operated in a xe2x80x9crecirculationxe2x80x9d or xe2x80x9cMax A/Cxe2x80x9d mode. In the manually controlled versions, the operator selects the desired mode, while in the automatically controlled versions, the selection is performed by a system controller based on various input parameters such as the desired cabin temperature, the measured cabin temperature, and so on. In any event, it is commonly recommended that the usage of cabin air recirculation be limited in order to more effectively purge odors, carbon-dioxide and smoke generated in the cabin, and in order to prevent intrusion of exhaust gases under certain conditions. Additionally, extended operation in the recirculation mode can tend to lower the relative humidity of the cabin air to an uncomfortable level.
At higher vehicle speeds, inlet air mixing and total air flow is also influenced by the flow of air around the vehicle body. Specifically, the air flow creates a positive pressure where ventilation air enters the vehicle and a negative pressure where ventilation air exits the vehicle. This can force a significantly higher amount of outside air through the ventilation system than ordinarily desired by the vehicle occupants.
The present invention is directed to an improved method for controlling inlet air mixing in a vehicle air conditioning system having an inlet air blower motor and an air inlet mixing device for admitting inlet air from outside and/or inside the vehicle cabin, wherein the blower motor and mixing device are controlled under predefined operating conditions to reduce compressor and blower motor power consumption and achieve performance improvements associated with cabin air recirculation while maintaining a predefined level of outside air flow in the inlet air mixture of the system. The total air delivered to the vehicle cabin is determined by the speed of the cabin air blower motor, the speed of the vehicle, and the position of the inlet air control door. Under high air conditioning load, the control of this invention adjusts the speed of the blower motor and the position of the inlet air mixing device as a function of both the vehicle speed and the selected blower motor speed so that the predefined level of outside air flow is preserved regardless of the vehicle speed and the selected blower motor speed. At low or no air conditioning load, when the position of the inlet air mixing device is set to provide only outside air flow, the control of this invention adjusts the speed of the blower motor to provide the desired air flow based upon the vehicle speed so that a predefined level of outside air is preserved regardless of vehicle speed and the selected blower motor speed.